AT Switch Settings
The IBM AT and clones do not have any DIP switch banks. The equipment
configuration is set by the SETUP program which is incorporated into the
DIAGNOSTICS diskette or changeable by another method (e.g., pressing
Ctrl+Alt+Esc while booting, depending upon the BIOS type).
See CMOS Memory for related information.
Older IBM-logoed ATs do have two system board options which are set by
A 3-pin Berg-strip connect is located on the front edge of the system
board labeled J18 affects the usage of system board RAM.
1 ║┌•┐║ ENABLE second bank of 256K (512K on system board)
3 ║ • ║
1 ║ • ║
3 ║└•┘║ DISABLE second bank of 256K (256K on system board)
A single slide switch near the back of the system board, labeled SW1,
selects the initial display adapter. This is used by the POST but has
meaning only when the CMOS Memory (which contains the full equipment list)
has lost power or has become invalid for some other reasons. In other
words, this switch indicates the primary display adapter that will be used
by the SETUP program.
║▐█▌║ ON ON (toward the rear) Color/Graphics Adapter (CGA) is primary
║ ║ OFF
║ ║ ON
║▐█▌║ OFF OFF (to the front) Monochrome Display Adapter (MDA) is primary
Note: The EGA reference contradicts this, saying to use OFF for a CGA
alone and ON for any other display adapter or combination of
adapters. You figure it out!
See Also: Equipment List